In some vehicles such as wheel loaders and bulldozers used as construction machines, a running-system hydraulic unit called ‘hydro-static transmission (HST)’ is installed between an engine and driving wheels. The running-system hydraulic unit includes a hydraulic pump configured to be driven by the engine, a hydraulic motor configured to be driven by pressure oil supplied from the hydraulic pump, and a pair of running-system oil passages connected between the hydraulic pump and the hydraulic motor for forming a closed circuit. Driving power of the hydraulic motor of the running-system hydraulic unit is transmitted to the wheels of the vehicle so that the vehicle can run.
In the case of a hydraulic vehicle driven by such a running-system hydraulic unit, specifications such as the maximum discharge amount of a hydraulic pump or the maximum capacity of a hydraulic motor are usually determined primarily based on the pulling force requirement. In this case, the maximum velocity of the vehicle is also inevitably determined by the specifications of the hydraulic pump and the hydraulic motor, and thus, market demands may not be satisfied.
In an attempt proposed to address the above-mentioned problem of the related art, a backup pump configured to be driven by an engine, and a backup motor configured to be driven by pressure oil discharged from the backup pump are installed, and it is configured such that output torque of the backup motor is added to output torque of a hydraulic motor through a speed reducer according to the operation of a backup selection valve. According to this technique of the related art, regardless of the specifications of a running-system hydraulic unit, the total output power can be increased by operating the backup selection valve, and thus the maximum velocity of a vehicle can be increased while maintaining a required towing force level (For example, refer to Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-open No. 11-350539